The present invention relates to a technique of magnifying images, which is applied, for example, to devices like mobile phones and PDAs (personal digital assistants) and to general software for displaying images.
Various methods for magnification of images have been proposed. Most common methods apply filtering to an image after (or while) up-sampling the image. Bilinear interpolation and bicubic interpolation are the most famous examples of such methods.
However, these methods tend to produce blurred magnified images. This is because high-frequency components are lost during image magnification.
To solve the problem, Non-Patent document 1 describes a method in which, after magnification of an image, an enhancement/estimation process is performed for lost high-frequency components to realize high-definition image magnification. Also, Non-Patent document 2 describes a method in which signal pattern after magnification is estimated on the basis of previously learned association between image signal pattern before being magnified and signal pattern after magnification. Alternatively, inventions similar to the present invention are described in Non-Patent document 3 and Patent document 1. Supposing that an image before magnification is a reduced and deteriorated version of a magnified image, pixel values for the magnified image are estimated. In the process, since each pixel value of the magnified image cannot be determined uniquely, a constraint condition is set for the search of a solution space, and the image is magnified by obtaining maximum (minimum) solutions under the constraint condition. Smoothness of neighboring pixels is often used for the constraint condition, and the resultant solution (i.e. magnified image) therefore tends to be blurred with high-frequency components lost. However, Non-Patent document 3 and Patent document 1 allow the solution search without losing high-frequency components by using constraint conditions utilizing learned data.
Prior arts that relate to the present invention include the techniques disclosed in Patent documents 1 and 2 and Non-Patent documents 1 to 3 shown below, for example.
[Patent document 1] JP 2000-339450 A
[Patent document 2] JP 2002-149105 A
[Non-Patent document 1] H. Greenspan, C. H. Anderson, and S. Akber, “Image Enhancement by nonlinear extrapolation in frequency space”, IEEE Trans. on Imag. Proc., vol. 9, no. 6, June, 2000.
[Non-Patent document 2] Tetsujiro Kondou, “Reality Creating Technique—Digital Reality Creation—”, The Journal of the Institute of Image Electronics Engineers of Japan, vol. 26, no. 6, 1997.
[Non-Patent document 3] Simon Baker and Takeo Kanade, “Limits on super-resolution and how to break them”, IEEE Trans. PAMI, vol. 24, no. 9, Sep., 2002.